1. Field of the Invention
The invention broadly relates to electrical energy storage devices and more particularly to a lithium electrode structure and a secondary electrochemical cell utilizing such an electrode.
2. Prior Art
Two approaches generally have been followed in the construction of a lithium electrode for use in an electrical energy storage device, such as a rechargeable battery, particularly one employing a molten salt electrolyte. In one approach, the lithium is alloyed with another metal such as, for example, aluminum to form a solid electrode at the operating temperature of the cell. In the other approach, liquid lithium is retained in a foraminous metal substrate by capillary action. Heretofore, the latter approach has been preferred because it offers higher operating cell voltages and therefore potentially higher battery energy densities. Certain problems are encountered, however, when it is attempted to retain molten lithium in a foraminous metal substrate. More particularly, most metals which are readily wetted by lithium are too soluble in the lithium to permit their use as the metal substrate, whereas most metals structurally resistant to attack by molten lithium are poorly wetted by the lithium when placed in a molten salt electrolyte.
It has been suggested that metals structurally resistant to attack by molten lithium may be wetted by immersion in molten lithium maintained at a hightemperature. However, the structure so wetted by lithium at these higher temperatures usually undergoes progressive de-wetting when used as the negative electrode in a secondary battery containing a molten salt electrolyte maintained at the substantially lower temperatures at which such a battery operates. Thus after operation of the battery for a number of cycles, it has been found that lithium no longer preferentially wets the substrate, the electrode progressively losing capacity. Various methods have been proposed in an attempt to overcome this problem. See, for example U.S. Pat. Nos. 3,409,465 and 3,634,144. None of the proposed methods have proven entirely satisfactory.
The use of a solid lithium alloy as taught by the prior art also is not without problems. More particularly, lithium-aluminum alloy, for example, is approximately 300 millivolts more positive than liquid lithium. Thus, electrochemical cells utilizing lithium-aluminum alloys as electrodes are not able to achieve the same potentials as those utilizing liquid lithium electrodes. Further, in a molten salt electrolyte, the lithium-aluminum alloy electrode expands and contracts greatly during charging and discharging of the electrochemical cell. Thus, it has been reported that the lithium-aluminum electrode may change in volume by as much as 200% during charging and discharging of the cell. Still further, lithium-aluminum alloys generally are limited to a lithium content of less than about 30 wt.%.
Various other materials have been suggested for use as an alloy with lithium to form a solid electrode. In U.S. Pat. No. 3,506,490, for example, it is suggested that the lithium be alloyed with either aluminum, indium, tin, lead, silver, or copper. However, none of these materials have been proven to be completely satisfactory. More particularly, these other suggested materials, such as tin and lead for example, form alloys containing lesser amounts of lithium than does aluminum, and thus have a still lower capacity (ampere-hours) per unit weight of alloy. Further, the potential of these other alloys compared with liquid lithium is more positive than that of a lithium-aluminum alloy; thus, alloys of such other materials are less desirable. Other patents relating to solid lithium anodes include U.S. Pat. Nos. 3,506,492 and 3,508,967.
As means of resolving some of the foregoing problems, U.S. Pat. No. 3,969,139 provides an electrode structure utilizing an alloy of lithium and silicon, this electrode being of particular utility as the negative electrode in a rechargeable lithium-metal sulfide molten salt cell. Such an electrode provides excellent lithium retention, significantly reduces corrosion, and provides twice the energy capacity of the lithium-aluminum electrode.
However, it was subsequently found that in electrochemically forming the lithium-silicon alloy electrode, not all the silicon was utilizable in the electrochemical forming process, thereby requiring a greater amount of silicon for a given ampere-hour capacity. Also, when utilized at high current densities, the lithium-silicon alloy electrode tended to become polarized during electrochemical transfer of lithium into and out of the electrode.
Co-pending U.S. patent application Ser. No. 715,358, filed Aug. 18, 1976, now U.S. Pat. No. 4,048,395, and assigned to the Assignee of the present invention, suggests that the foregoing problems are substantially overcome through the use of a ternary alloy of lithium, silicon and iron. The use of a lithium-silicon iron alloy permits substantially complete utilization of the silicon and further reduces the tendency of the electrode to polarize at high current densities. However, another problem has been found with the use of lithium-silicon alloys which is not overcome by the addition thereto of iron. Specifically, it has been found that during cycling of a lithium-silicon electrode that the silicon tends to migrate into the metal substrate supporting structure causing the metal to become brittle, lose its structural integrity and gradually disintegrate into small particles. Accordingly, the need still exists for an improved lithium electrode which retains the advantageous features of the lithium-silicon alloy electrode while at the same time minimizing or eliminating the disadvantageous features thereof.